1. Field of the Invention
The present invention relates generally to electronically beamsteered phased array radar antennas. More particularly, the present invention relates to the use of true time delay beamsteering signals in phased array radar antennas.
2. Description of Related Art
An electronically beamsteered phased array is an antenna system comprising individual fixed antenna elements in which the relative phases of the signals respectively radiated or received by the antenna elements control the effective beam pointing direction. A typical phased array antenna system 10 is shown in FIG. 1. The antenna 10 includes a number of phase shifters 12. Each phase shifter 12 provides signals to and receives signals from its corresponding antenna element 14. A pointing angle 15 is established by imparting an appropriate phase shift to the transmit and receive signals in each phase shifter 12. An RF wavefront 18 represents a line along which signals transmitted from each of the antenna elements 14 will line up in phase. A beam pointing direction 16 is perpendicular to the RF wavefront 18. The beam pointing direction 16 and RF wavefront 18 define a beam pointing angle 15 relative to the plane 19 of the antenna elements 14. An effective beam pointing angle is established for receive signals by applying an appropriate phase shift to the signals as they are received by elements 14. The beamsteering effect on both transmit and receive is substantially the same as that produced by physical repositioning of the antenna elements in a mechanically scanned antenna.
The operation of a prior art phased array antenna utilizing conventional phase shifters can be understood by reference to FIG. 2(a). The exemplary antenna system 20 includes a number of phase shifters 22 imparting appropriate phase shift to the signals transmitted by or received from their corresponding antenna elements 24. The different phase shift applied by each phase shifter 22 provides a beam pointing direction 25 at a desired beam pointing angle 23 as previously discussed. A portion of an exemplary radar signal is also shown in FIG. 2(a). The radar signal includes a pulse modulation envelope 27 superimposed on a composite RF carrier signal 28. The different phase shift imparted to signals from each phase shifter 22 results in a number of distinct phase shifted carrier signals 26. For purposes of clarity only the portion of each carrier signal 26 which falls within the time duration of a single exemplary modulation pulse is shown. The various phase shifted versions of the carrier signals 26 combine to produce the composite carrier signal 28. Line 29 is perpendicular to the beam pointing direction 25 and defines an exemplary RF wavefront. Along the wavefront all of the various phase shifted versions of the RF carrier signal 26 are aligned in phase and combine to produce the peak of the signal 28. However, since each phase shifted signal 26 takes a different amount of time to arrive at line 29, the composite carrier signal 28 is spread out in time and the carrier signal energy is not concentrated within the time duration of the pulse as desired. The pulse modulation envelope 27 therefore has a greater time duration than the pulse itself.
Present phase shifting techniques are capable of precisely establishing a particular beam pointing direction only over a relatively limited RF carrier signal frequency range. The appropriate phase shift applied in each phase shifter 12 is a function of RF carrier frequency as well as desired beam pointing angle. Frequency components of the radar signal which are higher or lower than the carrier frequency are therefore also shifted in phase by the same fixed phase shift.
The frequency dependency of the phase shift tends to limit the instantaneous bandwidth over which phased array radars can effectively operate. The instantaneous bandwidth is usually defined as the frequency range occupied by a particular signal at a given instant in time. For example, a narrow pulse of 1 ns is considered to have an instantaneous bandwidth of about 1 GHz. Instantaneous bandwidth should be distinguished from tuning bandwidth, which is typically defined as the total frequency range over which a system can be tuned or operated. The instantaneous bandwidth in a phased array antenna system using conventional phase shifters is limited because a fixed phase shift at a particular carrier frequency and beam pointing angle results in an excessive total phase shift at higher frequencies, and an insufficient total phase shift at lower frequencies. The greater the deviation in frequency from the carrier frequency, the greater the deviation in beam pointing direction. The transmitted and received signals are spread and thereby distorted by deviation in beam pointing direction. Currently available phase shifting techniques therefore significantly limit the instantaneous bandwidth over which these phased array radars can operate.
True time delay beamsteering has been used to alleviate the limited instantaneous bandwidth problem associated with conventional phase shifters. Delaying the arrival of the radar signal at the antenna has an effect similar to that of phase shifting, and serves to establish a beam pointing direction. In true time delay beamsteering, however, the conventional phase shifter discussed above is replaced with a true time delay device which can provide both phase and time delay. The true time delay device may incorporate a number of switchable fiber optic delay lines or other fiber optic components. A fiber optic cable of the appropriate length is typically switched into the transmit or receive signal path to generate the required time delay between signals corresponding to each of the antenna radiating elements. U.S. Pat. No. 5,051,754, assigned to the present assignee, discloses a true time delay phased array utilizing a photonic true time delay circuit for both the transmit and receive signal paths. The contents of U.S. Pat. No. 5,051,754 are incorporated herein by reference.
The effect of using a true time delay device to provide proper phase and time delay is shown in FIG. 2(b). An exemplary antenna system 30 includes a number of true time delay devices 32 which send and receive signals from their corresponding antenna elements 34. The true time delay devices provide appropriate time and phase delay to establish a beam pointing direction 35 at a desired beam pointing angle 33. Beam pointing direction 35 is perpendicular to an exemplary RF wavefront defined by line 39. Since the proper time delay has been applied to the individual carrier signals 36 from each of the antenna elements 34, each of the signals 36 line up in time within an exemplary modulation envelope 37. The time duration of modulation envelope 37 is therefore the same as the time duration of the pulse applied to each carrier signal 36. The signals 36 combine to form a composite carrier signal 38 which has a constant amplitude. The composite carrier signal energy is concentrated within the modulation envelope 37. The ideal amount of time delay can be determined independent of frequency to thus avoid the signal spreading problems associated with conventional phase shifters.
In a true time delay beamsteered phase array such as that shown in FIG. 2(b) the signal 36 transmitted or received by each of the n antenna elements 34 is typically delayed by a time (n-1)T in order for the signal wavefronts to line up in a particular beam pointing direction .theta.. This will result in the transmitted or received signals forming a coplanar wave in the desired beam pointing direction. The time delay T is determined for each of the true time delay devices in accordance with the following equation. ##EQU1##
In the above equation, m is the total number of antenna elements, d is the distance between adjacent elements, and v is the velocity of light. The time delay T is therefore independent of frequency. A true time delay beamsteered phased array is preferably capable of providing these time delays for both transmit and receive signals.
In a true time delay beamsteered system, the appropriate time delay as defined by the above equation is provided to the transmit and receive signals using fiber optics or other delay techniques. A similar fiber optic array, or the like, is used to generate the time delays for each of the antenna elements in order to establish a coplanar wavefront in a desired beam pointing direction. The bandwidth of the true time delay signal is therefore limited to the bandwidth of the delay line or other components utilized in the true time delay beamsteering device.
The full advantages of true time delay are thus not obtained under current practice. Most fiber optic components currently available can operate adequately up to a frequency of approximately 5 GHz. Although certain fiber optic components operating at frequencies up to 10 GHz are commercially available, at higher frequencies performance generally degrades or the cost to achieve good performance goes up significantly. At frequencies of about 18 GHz or higher, currently available fiber optic components typically have poor performance and therefore are not generally considered suitable for steering a phased array antenna.